230 GENERAL DISTRIBUTION OF JURASSIC STRATA IN RUSSIA. 
gland and the north of France, with valleys of clay and lias. In their place we 
found low masses only of slightly coherent shale, with sands and concretions ; 
which being superposed to the Palaeozoic rocks, and containing Ammonites and 
Belemnites, clearly belonged to some member of the great secondary division of 
deposits. 
In our first exploration of the Volga below Kostroma, where such Ammonite 
beds occur, we were indeed disposed to refer some of them to the Lias, because 
we were then led to think (as previously stated, p. 179), that the red deposits in 
that neighbourhood might represent the Keuper of Germany and France. More 
extended researches, however, induced us to adopt other views ; for whilst we 
could obtain no sort of proof that any of the red deposits in question belonged to 
the Trias, so we found distinct evidence, that the overlying strata of which we are 
now treating, though resembling the Lias in mineral character, were, in fact, by their 
organic remains, the representatives of the Oxfordian or middle oolite. 
All these detached masses of Jurassic (Oxfordian) strata have a surprising uni- 
formity of character, whether found near Moscow, on the Oka, on the Lower Volga, 
or in the remote district of Ust Sisolsk, in the government of Vologda. To these 
districts, examined by ourselves in the years 1840 and 1841, we may add three still 
more distant and northern tracts, where strata of the same character, and containing 
the same fossils, have been discovered through the enterprize of Major Strajevski, 
an officer ol the Imperial School of Mines, by M. Ruprecht, the botanist, and since 
we left the country, by Count Keyserling, our own associate. The first of these is on 
the east flank of the North Ural, inN. lat. 64°, and on the banks of the little rivers 
Tchol and Tolya ; the latter in N. lat. 68°, on the shores of the White Sea, east of 
Mezene (see Map). The third is in the depression by which the great river Petchora 
passes to the Icy Sea, and has been made known to us whilst we write by Count 
Keyserling, who in the past summer (1843) led an expedition into those wilds, in- 
habited only by Samoyedes, and which were previously an entire blank, even upon all 
Russian maps. We shall hereafter, as well as in our Introduction, point out the 
agreement of the general succession in this northern region with the strata in other 
parts of Russia. Of the Jurassic deposit which now concerns us. Count Keyserling 
thus speaks : — “ It is a widely spread and low formation in the north, which forms 
the ‘ tundra ’ or mosses, and occurring in all the depressions of the more ancient 
rocks, occupies extensive, marshy, wooded tracts. It is, for the most part, com- 
posed of grey or black clay, with calcareo-arenaceous concretions, disposed along 
